1. Field of the Invention
This invention relates to a circuit for injecting data containing error into an electronic circuit for which data containing errors may not normally be transmitted. In particular, an error injection circuit for testing a fiber optic distributed data interface (FDDI) network is described.
2. Background of the Invention
FDDI token ring networks are well known to those skilled in local area network technology. FDDI is a result of American National Standards Institute Committee X3T9. It supports a variety of networks configured in a variety of topologies and provides for secure transmission across long distance links.
The FDDI interface defined in X3T9.5 relates to the lower layers of the Open Systems Interconnection/International Organization for Standardization (OSI/ISO) model. Additional information on the ANSI X3T9.5 standard can be found in ISO 9314-2 (1989) "Fiber Distributed Data Interface (FDDI) Media Access Control (MAC)" and ISO 9314-1 (1989) "Fiber Distributed Data Interface (FDDI) Physical Layer Protocol (PHY)". Further information can be found in U.S. Pat. No. 4,979,167 and 4,890,304, both assigned to Advanced Micro Devices, Inc. These patents relate to specific implementations of FDDI networks.
As well known by those skilled in FDDI network technology, information to be transmitted over the FDDI network resides in a memory, or RAM buffer. When data is to sent out, a RAM buffer controller (RBC) and data path controller (DPC) work simultaneously to send the data to a fiber optic media access controller (FORMAC). The FORMAC sends the data to be transmitted to an Encoder/Decoder (ENDEC). The ENDEC then sends the data to the fiber optic transmitter.
The FORMAC performs media access control (MAC) layer protocol for the FDDI network. The FORMAC determines when a node can get access to the network and implements logic required for token handling and address recognition generation. The FORMAC controls the transmission of data by an internal transmit state machine that implements the FDDI MAC functions. Before the data is transferred, the FORMAC insures that the data (frame) to be sent corresponds to the FDDI standard for sending a frame. The standard states the maximum frame length, what information the frame should contain, and amount of preamble needed in front of the frame before a frame transmission can take place. The FORMAC can also generate FDDI command symbols. The FORMAC insures that only valid frames and symbols will be transmitted onto the optical media.
The FORMAC is also involved in the reception of data (frames). If the incoming frames contain errors, the FORMAC has the responsibility of detecting these errors and logging them so that the higher layers of software can tell if any errors occurred during frame transmission and reception.
Since the FORMAC is only able to send valid frames and symbols, a problem arises that it is impossible to verify that the receiving FORMAC and its supporting chips can detect and recover from errors. To verify correct operation of this logic, it is necessary to have some means of injecting data containing errors onto the fiber optic media. Existing devices to perform error injection functions are stand-alone devices and cannot operate as network node with a functional MAC layer.